This invention relates to the art of injection molding or casting of thixotropic alloys and, particularly, to an improved structure of an alloy or composite feed material for use in producing a thixotropic alloy, and a method of producing the feed material, and more particularly to a feed material and method for producing a feed material for use in forming a thixotropic alloy.
It is known, as disclosed for example in U.S. Pat. No. 4,694,881 to Busk, U.S. Pat. No. 4,694,882 to Busk and U.S. Pat. No. 5,040,589 to Bradley, the disclosures of which are hereby incorporated herein by reference for background purposes, thixotropic alloys can be produced by processing solid particles of a metal alloy in an extruder, such as a screw extruder. In the earlier of the Busk patents, the solid particles are heated to a temperature above the liquidus temperature of the alloy, and the resulting molten mass is subsequently cooled to a temperature between the solidus and liquidus temperatures and subjected to shearing to break the dendritic structure formed during solidification. The resulting liquid-solid mixture of a thixotropic alloy is injected into a mold to form a molded product. In the more recent Busk patent, and in the patent to Bradley, the particles of feed material are heated to a temperature between a solidus and liquidus temperatures, whereby complete melting of the feed material does not take place.
In both of the Busk patents, the feed material is disclosed as being particles or chips of a metal alloy which are described as being of a convenient size for handling and as having an irregular, non-uniform shape, and the patents refer to the material being ground to a 50 mesh size. The Bradley patent refers to pellets, chips or powder, but not to a size therefor and, as disclosed in U.S. Pat. No. 5,577,546 to Kjar, et al., the disclosure of which is hereby incorporated herein by reference for background purposes, the particulate feed material for use in the foregoing processes are particles shaped such that the ratio of the length of the largest dimension to the effective diameter of the particle is in the range of 1.0 to 4.0 and, most preferably, 1.2 to 2.0 with a maximum dimension in the range of 0.5 to 5.0 and, preferably, in the range of 1.0 to 3.0 millimeters. The particles in Kjar, et al. preferably have an ovoid shape or a teardrop shape. In any event, irregular shaped chips such as those referred to in the Busk patents are prone to clog the feed hopper and seize the screw extruder in which the particles are processed to produce a thixotropic alloy. Moreover, irregular particle profiles do not exhibit good packing characteristics relative to one another and this can result in difficulty in achieving adequate heat transfer rates to cause the partial melting of the metal particles and can make it difficult to control the process temperature. While spherical particles provide an optimum particle profile for minimizing the foregoing problems, the cost of producing spherical particles is unacceptably high, and the process for producing such particles can be dangerous. Similarly, the ovoid or teardrop shaped particles described in Kjar, while providing somewhat less efficient packing, feeding and heat transfer characteristics, compared to spherical particles, also have unacceptably high production costs, and require a possibly dangerous production process.
In accordance with the principal aspect of the present invention, a particle structure or profile for feed material used in producing a thixotropic alloy is provided which minimizes or overcomes the foregoing problems encountered in connection with the particulate feed material heretofore available. Accordingly, the feed material particles of the present invention have a substantially uniform particle contour or profile which provides several advantages over past types of thixotropic alloy configurations. The uniform particle contour or profile of the feed material particles avoids a particle contour or profile with irregular or undesired shapes on the particles which can interfere with achieving good packing of the particular material and which can cause jamming or seizing of the screw extruder. The uniform particle contour or profile of the feed material particles also facilitates in obtaining a desired heat transfer rate and a desired control of the process temperature when forming the thixotropic alloy.
In one particular aspect of the present invention, the feed material particles are solid cylinders having end faces that are substantially perpendicular to the axis of the cylinder. The cylindrical configuration advantageously provides a uniform particle contour or profile. The cylindrical design has a particle contour or profile which achieves good packing of the particular material and avoids jamming or seizing of the particle mover. The cylindrical design also facilitates improved heat transfer rates and desired control of the process temperature when forming the thixotropic alloy. The solid cylindrical particles can be readily produced by continuously extruding one or more columns of the metal alloy or composite with the rate of movement of the column or columns coordinated with the speed of a cutting device so that the particles are of substantially uniform length and configuration. Typically, the cylindrical thixotropic alloy particles have a length to diameter ratio of about 0.8:1 to about 4:1 and, preferably about 1:1 to about 2.5:1, and more preferably about 1.5:1 to about 2:1. The maximum length of the cylinder is generally less than about 1.0 inch, preferably about 0.1 to about 0.5 inch, and more preferably about 0.15 to about 0.25 inch. Furthermore, the maximum diameter of the cylinder is generally less than about 0.75 inch, preferably about 0.05 to about 0.4 inch, and more preferably about 0.075 to about 0.125 inch.
In accordance with another aspect of the present invention, the particles are formed from a cylindrical extrusion which has been cut so that at least one end face of the particles is oblique to the axis of the particle. This modified cylindrical configuration also advantageously provides a uniform particle contour or profile. This modified cylindrical design also has a particle contour or profile which excludes flattened or pointed portions of the longitudinal outer surface of the particle thereby achieving good packing of the particular material and/or avoiding jamming or seizing of the a particle mover. This modified cylindrical design further facilitates improved heat transfer rates and desired control of the process temperature when forming the thixotropic alloy. The solid modified cylindrical particles can be readily produced by continuously extruding one or more columns of the metal alloy or composite with the rate of movement of the column or columns coordinated with the speed of a cutting device so that the particles are of substantially uniform length and configuration. In one embodiment, one end of the particle is substantially perpendicular to the axis of the particle and the other end is not perpendicular to the axis of the particle. In one aspect of this embodiment, the end that is not perpendicular to the axis of the particle is substantially uniformly sloped. The angle of slope is typically between about 5-75xc2x0, and preferably about 30-60xc2x0. In another embodiment, both ends of the particle are not perpendicular to the axis of the particle. In one aspect of this embodiment, the two ends of the particle slope toward one another. In one design, the two ends have angles of slope which are substantially equal. In this design, the side profile of the particle has an isosceles triangle shape. In another design, the side profile of the particle is trapezoidal. In still another design, the side profile of the particle is substantially triangular and the two ends have an angle of slope which is different. In still another design, a portion of the two ends merge at least at one surface of the particle. In still yet another design, the ends are substantially uniformly sloped. The angle of slope is typically between about 5-75xc2x0, and preferably about 30-60xc2x0. In still another embodiment, at least one end has a sloped portion and a portion that is substantially perpendicular to the axis of the particle. In still yet another embodiment, the particles have a maximum length to maximum diameter ratio of about 0.8:1 to about 4:1 and, preferably about 1:1 to about 2.5:1, and more preferably about 1.5:1 to about 2:1. The maximum length of the cylinder is generally less than about 1.0 inch, preferably about 0.1 to about 0.5 inch, and more preferably about 0.15 to about 0.25 inch. Furthermore, the maximum diameter of the cylinder is generally less than about 0.75 inch, preferably about 0.05 to about 0.4 inch, and more preferably about 0.075 to about 0.125 inch.
In accordance with yet another aspect of the present invention, the particles are formed from a substantially cylindrical shaped extrusion of metal which has been cut to form the substantially uniformly shaped particles. During the cutting process, one or more substantially cylindrical shaped extrusions of metal can be directed to the cutter. In one embodiment, the substantially cylindrical shaped extrusion of metal is fed through a shear cutting wheel which shears the cylindrical shaped extrusion as the shear wheel rotates while the extrusion is fed through one or more openings in the shear wheel. In another embodiment, the substantially cylindrical shaped extrusion of metal is fed to a shearing ledge to be shear cut by a blade. In another embodiment, the substantially cylindrical shaped extrusion of metal is fed to a cutter that has one or more teeth used to periodically cut the extrusion as the extrusion passes through the cutter.
In accordance with still yet another aspect of the present invention, the thixotropic alloy can be produced using feed material according to the present invention by heating and shearing the feed material, such as in a screw extruder, to produce a mixture of solid particles within a liquid metal, and an article of manufacture can be provided by injecting this mixture into an article mold and at least partially solidifying the mixture in the mold prior to removing the article from the mold. The feed material of the present invention may be of any desired metal alloy or composite thereof such as, for example, metals and alloys based on lead, aluminum, zinc, magnesium, copper, and iron with the preferred material being alloys of magnesium.
It is accordingly a principal object of the present invention to provide an improved feed material for use in producing a thixotropic alloy.
Another object of the present invention is the provision of a feed material for the foregoing purpose having a solid feed material which a profile to minimize blockage and/or seizure of a particle transport mechanism.
Yet another object of the present invention is the provision of a feed material having improved packing characteristics.
Still another object of the present invention is the provision of a feed material having improved heat transfer and control characteristics which promotes the formation of a thixotropic alloy.
Still yet another object of the present invention is the provision of a feed material having an essentially circular cross-sectional shape.
A further object of the present invention is the provision of a feed material having a substantially uniform shape.
Yet a further object of the present invention is the provision of a feed material having a substantially cylindrical shape.
Still a further object of the present invention is the provision of a feed material having an isosceles shaped side profile and a substantially circular cross sectional shape.
Still yet a further object of the present invention is the provision of a feed material having a trapezoidal side profile and a substantially circular cross sectional shape.
Another object of the present invention is the provision of a feed material having a triangular side profile and a substantially circular cross-sectional shape.
Another object of the present invention is the provision of a method of producing feed material for the foregoing purpose.
Still another object of the present invention is the provision of a method for producing a thixotropic alloy using feed material according to the present invention.
Yet another object of the present invention is the provision of a method of producing an article of manufacture using feed material in accordance with the present invention.
These and other objects and advantages will become apparent to those skilled in the art upon the reading and following of this description taken together with the accompanied drawings.